Validating the return period of rainfall thresholds used for Extreme Rainfall Alerts by linking rainfall intensities with observed surface water flood events

In 2008, the Environment Agency and the Meteorological Office launched a pilot Extreme Rainfall Alert (ERA) service as a first step towards providing a warning for surface water flooding. An ERA is based on rainfall intensity thresholds of an average of 1-in-30 year storms, to represent rainfall thought likely to cause severe surface water flooding in urban areas. This research investigates whether this return period is appropriate for warning of surface water flooding by examining the intensity and return period of rainfall associated with observed surface water flood (SWF) events. Data from rainfall gauges and flood events were analysed which showed that most of the SWF events were associated with rainfall intensities of less than 1-in-10 year return period. Improvements in data recording could provide flood magnitudes and durations, which would facilitate better understanding of the relationship between flood magnitude and rainfall intensity and allow more informed debate about SWF warning thresholds.